This invention relates to a low energy method of making hexametaphosphates from sodium tripolyphosphate (STPP), tetrasodium pyrophosphate (TSPP), or trisodium phosphate (TSP). In particular, it relates to a method of making sodium hexametaphosphate (SHMP) from those phosphates when they are off-spec.
STPP is normally made by mixing together soda ash (Na2CO.sub.3) and/or caustic soda (NaOH) with phosphoric acid in a molar ratio of 5 sodium to 3 phosphorus, followed by removal of the free water, then dehydration of the bound water. Dehydration is accomplished by calcining without melting at a temperature of about 410.degree. to about 500.degree. C. In a large commercial facility for producing STPP, large quantities of off-spec STPP can be made.
Material can be "off-spec" for many reasons, but the most common reasons are that it is too finely powdered, too lumpy, does not contain the desired amount of Phase I content, has excessive pyrophosphate, short chain or insoluble metaphosphates, or a carbonaceous impurity content which produces an unacceptable color, or has a density which is too high or too low. This off-spec material cannot be easily sold commercially and must be otherwise disposed of.
At the present time there are a number of ways of disposing of off-spec STPP, none of which are entirely satisfactory. Off-spec STPP can be redissolved, hydrolyzed to orthophosphate, and reprocessed, but this treatment requires a great deal of energy to evaporate the water a second time. Furthermore, filtration and/or adsorption with carbon may be required to reduce high impurity levels. Another way of disposing of the off-spec STPP, when the off-spec STPP is too fine, is to compact the powder into larger flakes using large compacting machines. However, the properties of the compacted powder are not as good as on-spec STPP because the compacted powder particles tend to be too brittle and too dense. Compaction or agglomeration are unacceptable methods for dealing with STPP whose pyrophosphate, metaphosphate, or non-phosphate impurity levels are too high. A third method of disposing of off-spec STPP is to recycle it into the calciner and spray it with water or sodium phosphate to agglomerate it. While in many cases this produces an adequate product, it reduces the capacity of the plant to produce STPP because it reduces the quantity of fresh feed that the calciner can accept.